Multicolumn table in LaTeX

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\begin{longtable}{|ll|ll|}
\caption{Pomiary z diodą 1A (druga lekcja)}\vspace{1mm}\\

\hline
\multicolumn{2}{|c|}{$\text{U}(\SI{}{V})$} & \multicolumn{2}{c|}{$\text{I}(\SI{}{A})$}  \\  \multicolumn{1}{|c}{wartość} & \multicolumn{1}{c|}{$\pm$} & \multicolumn{1}{c}{wartość} & \multicolumn{1}{c|}{$\pm$}   \\ \hline
 0.716 & 0.006 & 0.018 & 0.011 \\
    0.762 & 0.006 & 0.051 & 0.012 \\
     0.768 & 0.006 & 0.062 & 0.012 \\
    0.787 & 0.006 & 0.096 & 0.012 \\
    0.794 & 0.006 & 0.122 & 0.013 \\
    0.801 & 0.007 & 0.147 & 0.013 \\
   0.809 & 0.007 & 0.179 & 0.014 \\
   0.814 & 0.007 & 0.216 & 0.015 \\
    0.820 & 0.007 & 0.258 & 0.016 \\
   0.823 & 0.007 & 0.307 & 0.017 \\
   0.826 & 0.007 & 0.356 & 0.018 \\
   0.829 & 0.007 & 0.399 & 0.018 \\
    0.830 & 0.007 & 0.453 & 0.020 \\
    0.830 & 0.007 & 0.494 & 0.020 \\
     0.831 & 0.007 & 0.560 & 0.022 \\
    0.831 & 0.007 & 0.601 & 0.023 \\
    0.830 & 0.007 & 0.666 & 0.024 \\
   0.831 & 0.007 & 0.710 & 0.025 \\
    0.831 & 0.007 & 0.756 & 0.026 \\
    0.832 & 0.007 & 0.819 & 0.027 \\
   0.832 & 0.007 & 0.852 & 0.028 \\
   0.837 & 0.007 & 0.945 & 0.029 \\
    0.816 & 0.007 & 0.600 & 0.022 \\
  0.814 & 0.007 & 0.511 & 0.021 \\
    0.813 & 0.007 & 0.446 & 0.019 \\
   0.808 & 0.007 & 0.361 & 0.018 \\
 0.805 & 0.007 & 0.278 & 0.016 \\
   0.809 & 0.007 & 0.275 & 0.016 \\
    0.789 & 0.006 & 0.151 & 0.014 \\
  0.825 & 0.007 & 0.387 & 0.018 \\
  \hline
   \multicolumn{2}{|c|}{$\text{U}(\SI{}{V})$} & \multicolumn{2}{c|}{$\text{I}(\SI{}{A})$}  \\ \multicolumn{1}{|c}{wartość} & \multicolumn{1}{c|}{$\pm$} & \multicolumn{1}{c}{wartość} & \multicolumn{1}{c|}{$\pm$}   \\ \hline
   0.821 & 0.007 & 0.404 & 0.019 \\
  0.822 & 0.007 & 0.429 & 0.019 \\
    0.824 & 0.007 & 0.455 & 0.020 \\
   0.827 & 0.007 & 0.513 & 0.021 \\
   0.818 & 0.007 & 0.568 & 0.022 \\
   0.812 & 0.007 & 0.425 & 0.019 \\
  0.830 & 0.007 & 0.641 & 0.023 \\
   0.841 & 0.007 & 0.825 & 0.027 \\
   0.809 & 0.007 & 0.524 & 0.021 \\
    0.808 & 0.007 & 0.352 & 0.018 \\
    0.371 & 0.004 & 0.953 & 0.030 \\
   0.380 & 0.004 & 1.098 & 0.032 \\
  0.388 & 0.004 & 1.256 & 0.036 \\
   0.392 & 0.004 & 1.330 & 0.037 \\
  0.397 & 0.004 & 1.430 & 0.039 \\
   0.406 & 0.005 & 1.606 & 0.043 \\
   0.415 & 0.005 & 1.771 & 0.046 \\
    0.420 & 0.005 & 1.889 & 0.048 \\
    0.426 & 0.005 & 2.020 & 0.051 \\
    0.383 & 0.004 & 1.374 & 0.038 \\
   0.363 & 0.004 & 1.002 & 0.031 \\
   0.414 & 0.005 & 1.767 & 0.046 \\
    0.377 & 0.004 & 1.228 & 0.035 \\
    0.347 & 0.004 & 0.792 & 0.026 \\
    0.316 & 0.004 & 0.413 & 0.019 \\
    0.371 & 0.004 & 1.032 & 0.031 \\
    0.345 & 0.004 & 0.689 & 0.024 \\
   0.303 & 0.004 & 0.272 & 0.016 \\
   0.385 & 0.004 & 1.187 & 0.034 \\
   0.420 & 0.005 & 1.793 & 0.046 \\
   0.355 & 0.004 & 0.874 & 0.028 \\
   0.408 & 0.005 & 1.626 & 0.043 \\
   \hline
\end{longtable}
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